A conventional aqueous ink composition comprising a water soluble dyestuff such as direct dye, acid dye and basic dye has various advantages such that, when it is written or printed on a sheet of paper, it does not blur, or causes little set-off, although the printed image has poor water resistance (cf. Japanese Patent Kokai Publication Nos. 77706/1978, 75465/1980, 2361/1981, 133377/1981, 133378/1981, 136861/1981, 139568/1981, 139570/1981 and 63368/1982). It is, therefore, highly desired to provide an aqueous composition which can produce an image with good water resistance when used as an ink composition for an aqueous fountain pen and an aqueous ball point pen.
To impart good water resistance to an aqueous ink composition, it is proposed to disperse a finely ground pigment in an aqueous resin emulsion, to suppress excessive water solubility of the direct or acid dye by adding as a counter ion a basic dye or a high molecular weight amino compound, or by linking or adding a group reactive with paper material or a water dispersible polymeric chain to the water soluble dye.
However, the ink composition containing the dispersed pigment particles has high viscosity and its writing characteristics are decreased by sedimentation of the particles. Other proposals are still unsatisfactory.
It has been tried by the present inventors to incorporate the reactive group or the water dispersible polymeric chain to C. I. Direct Black 154, which is widely used as a component of the aqueous ink composition and disclosed in Japanese Patent Kokai Publication (unexamined) No. 11973/1981 or its analogous dyestuffs. However, it is difficult to prepare C. I. Direct Black 154 with a stable composition so that it is very troublesome to control the quantitative relationship between said dyestuff and a reagent to be reacted. Further, said dyestuff contains undesirable impurities which would deteriorate writing characteristics and water resistance of the printed image. Thus, reproducibility of stability of the ink composition and water resistance of the printed image is hardly achieved. In addition, said dyestuff is of the tolidine type, which is known to be undesirable as a dyestuff.